Although he’d warned her exactly what would happen if he was killed, the dreaded knock on the door was more than Pfc.

Collin Mason’s wife could bear.

The 20-year-old Staten Island soldier was dead, they had come to tell her, but she knew it before she even opened the door.

“He told me what would happen,” heartbroken newlywed Cynthia I.

Mason, 21, told The Post by phone from Fort Hood, Texas. “When I looked through the peephole, I knew.” The two solders in dress uniforms informed her that her husband of 8½ months had been killed Sunday by a rocketpropelled grenade at a checkpoint outside Fort Taji north of Baghdad.

But Mason’s death, caused by “indirect fire,” is continuing to be investigated, Army officials said.

She said she had spoken to her husband, who grew up in the South Beach section, the day before he was killed and he sounded strong.

“He said he was going on a mission. He said he loved me and he said he’d call me tomorrow,” she said.

The call never came.

“He’s resting now. I know he’s my hero,” she said.

In Staten Island, Mason’s mother, Cynthia Boone-Mason, said her son was “extremely proud” to be a soldier.

“He was GI Joe. He was always GI Joe. That says it all,” she said.

Mason’s sister, Cheshire Mickens, 27, was devastated.

“I lost my brother. He was not even legally able to drink, but he could die,” she said.

“My daughter will never know her uncle.” Mason, a member of the Army’s 166th Armored Battalion, had been deployed to Iraq in December, a little more than a month after he and his wife wed at Staten Island Borough Hall, she said.

Mason attended Curtis HS, where he met Cynthia, the sister of his best friend, Francisco Martinez.

Mason later completed a GED, and joined the Army in August 2004. He was always a military buff, his wife said. His favorite movies were “Black Hawk Down” and “Basic.” But he was not always enthusiastic about being in Iraq.

“He used to say he hated Iraq because he hated the sand,” said Martinez.

“I told of him to think of it as a beach with no water.” However, when Martinez would exchange instant messages his pal, Mason would be upbeat.

“When I would ask ‘How are we doing?’ Collin would reply, ‘We are winning.’ ”